The present invention relates to thermal transfer printing, and, more specifically, to improved thermal transfer printing processes wherein transfer of the ink to the receiver sheet is enhanced by an electric or magnetic field.
Thermal printing is a no nonipact printing process that enables formation of high resolution images. These printing processes are simple, offer low noise levels, and are very reliable over extended usages. Thermal printing processes may be classified into three categories. Direct thermal printing entails the imagewise heating of special papers coated with heat sensitive dyes, such that an image forms in the heated areas. Another method of thermal printing is known as the dye transfer or dye sublimation technique, and operates by heating a transfer element coated with a sublimable dye, which transfer element is not in contact with the receiving sheet. When the transfer element is imagewise heated, the dye sublimates and migrates to the receiver sheet, which possesses a polymeric coating into which the dye diffuses, forming an image. A third method of thermal printing is known as thermal transfer printing. The thermal transfer printing process entails imagewise heating of a transfer element containing ink, which transfer element is in intimate contact with the heater on one surface and the receiving sheet on the other surface. Imagewise heating of the transfer element affects the ink in such a way as to cause it to transfer from the transfer element to the receiving sheet, thereby resulting in image formation. Thermal transfer printing methods generally employ uncoated plain papers, which enables prints with acceptable appearance and excellent archival properties. In addition, the thermal transfer printing method can be employed for color printing applications by using transfer elements of the desired color or colors.
Thermal transfer printing processes generally employ a thermal printhead, a transfer element, and a receiver sheet. The side of the transfer element containing the ink is placed in contact with the receiver sheet, and heat originating from the printhead is then applied to the transfer element. Heat conducted through the element increases the temperature of the ink, which can cause it to melt, soften, decrease in viscosity, or otherwise undergo a transition that enables the ink to transfer to the receiver sheet. After the receiver sheet and transfer element are separated, an image remains on the receiver sheet. An alternative method of heating the transfer element, known as resistive heating, employs an array of electrodes instead of thermal printhead to generate a current between the electrodes and a grounded conductive layer in the transfer element. This method is described in the IBM Journal of Research & Development, Vol. 29, No. 5, 1985, the disclosure of which is totally incorporated herein by reference. Additional information concerning thermal transfer printing processes is disclosed in Thermal Transfer Printing: Technology, Products, Prospects, published by Datek Information Services, P.O. Box 68, Newtonville, Mass., the disclosure of which is totally incorporated herein by reference.
The processes of the present invention enhance the thermal transfer printing process by assisting the transfer of the ink to the receiver sheet by means of an electric or magnetic field. Assisting the transfer processes enables more rapid printing processes, since the ink is drawn toward the receiver sheet. Assisting transfer also enhances the formation of images on rough paper, since the field attracts or pushes the ink into the depressions on the surface of a rough receiver sheet. In addition, field assisted thermal transfer printing processes enhance printing with multiuse transfer elements, especially those as described in copending application U.S. Ser. No. 454,800, the disclosure of which is totally incorporated herein by reference. The lifetime of a multi-use thermal transfer element is improved by carefully metering the amount of ink released during each use, so that only the required amount of ink is released from the transfer element and the remaining ink is available for subsequent imaging processes using the transfer element. Selective application of a magnetic or electric field to a multi-use transfer element can meter the amount of ink released for each image formed by either enhancing or restricting ink release. Further, field assisted thermal transfer printing processes in which multi-use transfer elements are employed enables the formation of images having a "gray scale" of image density. By gray scale, it is meant that the image density can be varied along a continuum from no image at all to maximum image density.
The thermal transfer printing process has been disclosed in, for example, U.S. Pat. No. 3,441,940 and U.S. Pat. No. 3,745,586, the disclosures of each of which are totally incorporated herein by reference. In addition, augmented thermal transfer printing processes are known. For example, U.S. Pat. No. 3,989,131 discloses a pressure assisted thermal transfer printing process employing an electrothermic printing unit for writing dot matrix characters on a printing line of recording medium by means of an electrothermal printing head which is continually movable along the printing line. Pressure is interposed between the head and the recording medium, pressure means being provided for pressing the printing elements against the transfer element and the receiver sheet. In addition, U.S. Pat. No. 4,541,042 discloses a transfer recording process assisted by a solvent, wherein a receiving medium such as paper and an ink transfer sheet are placed in contact between a platen and a thermal head, and a liquid, volatile solvent is applied to the paper. The solvent enables high speed thermodissolving transfer of the ink to the paper by heating selected areas to form an image.
Further, U.S. Pat. No. 4,525,722 discloses a thermal transfer printing process assisted by chemical heat amplification, wherein some of the heat necessary for melting and transferring the ink from a solid fusible layer in a ribbon to a receiving medium is provided by an exothermic reaction involving an exothermic material contained in a layer in the ink ribbon. Also, U.S. Pat. No. 4,549,824 discloses a thermal transfer printing process aided by an exothermic reaction, wherein an aromatic azido compound is added to the ink, said azido compound being one that exotherms at the conditions of thermal ink transfer. In addition, U.S. Pat. No. 4,550,324 discloses an ink transfer thermal printer utilizing a thermosensitive ink that is solid at normal temperatures, with selected portions of the ink being liquefied by heating and transferred onto recording paper. The printer can be of either contact or non-contact (ink jet) configuration, and eliminates the need to utilize disposable materials such as ink ribbons.
U.S. Pat. No. 4,567,489, discloses a thermal printhead for a thermographic printer having an electrically insulating substrate on which resistors are placed that form impression points and current supply and current discharge leads bonded to the resistors. The printhead includes a structure for forming a magnetic field that acts on the resistors in the immediate proximity of the resistors and along the resistor print line. The magnetic field is directed such that when the current flows through the resistors, the current paths are deflected upward into the upper part of the resistor on its outer surface. The single resistor impression points thus reach their highest temperature at the printing surface where they must deliver heat to the recording medium, which results in the heat needed for heating the resistor being supplied more quickly to the recording medium, thereby reducing the cooling time of the single resistor impression point so that a higher printing velocity can be attained with the thermal printhead.
Additionally, U.S. Pat. No. 4,510,511 discloses a picture recording method and apparatus using an ink containing an evaporable coloring matter, which enables printing on a medium without an ink ribbon. The special ink is supplied to an ink transporting means and then cooled below the melting point of the ink bonding agent. A discharge energy is applied, controlled according to the picture to be formed, which causes the coloring matter to fly to the recording medium opposite the transporting means. Essentially, the process entails fluidizing a marking material by heat, picking up the liquid marking material on a gravure type roll, and selectively transferring it to the receiving sheet by means of a high voltage field.
In addition, U.S. Pat. No. 4,803,119, the disclosure of which is totally incorporated herein by reference, discloses ink coating compositions for impact typewriter ribbons, which ink coatings comprise a sponge material having dispersed therein an ink comprising pigment particles and a dimer acid. Further, U.S. Pat. No. 3,348,651, the disclosure of which is totally incorporated herein by reference, discloses pressure sensitive ink transfer ribbons, tapes, and sheets having a microporous inking composition for use in typewriters, high speed printers, and optical scanning devices. The pressure sensitive ink transfer medium comprises a shock-absorbent base layer of an elastomeric polymer film having a high degree of resiliency in a direction normal to the plane of the film, an intermediate layer of a thin, non-elastic polymer film bonded to the base layer, and an inking layer bonded to the intermediate layer over substantially its entire working surface and comprising a substantially continuous film of a microporous inking composition. The microporous inking composition consists essentially of a uniformly blended mixture of an elastomeric polymeric binder, an inking compound comprising a non-aqueous, non-volatile ink carrier which is substantially insoluble in the elastomeric polymeric binder and which contains a high concentration of an ink pigment, and a finely ground microporous inorganic filler. Other patents, such as U.S. Pat. No. 3,287,153, U.S. Pat. No. 3,392,042, U.S. Pat. No. 3,484,508, U.S. Pat. No. 3,930,099, U.S. Pat. No. 4,321,286, U.S. Pat. No. 4,544,292, and U.S. Pat. No. 4,624,881, also disclose pressure sensitive porous marking ribbons filled with an exudable marking material. Of general interest is U.S. Pat. No. 2,940,847, which discloses improved methods and means for color electrophotography and includes transfer imaging using electromagnetic energy augmented by an electric field. In addition, U.S. Pat. Nos. 3,351,948, 3,847,265, 4,251,276, 4,414,555, 4,415,903, 4,603,986, 4,608,577, 4,762,734, 3,480,962, 4,128,345, 4,205,320, and 4,315,267 are of background interest.
Although the prior art processes are suitable for their intended purposes, a need continues to exist for improved thermal transfer printing processes. A need also exists for thermal transfer printing processes employing multi-use transfer elements in which the amount of ink released from the transfer elements is metered by means of a field. In addition, a need exist for thermal transfer printing processes that enable formation of images within a gray scale of image density. Further, a need exists for thermal transfer printing processes in which printing speed is augmented or enhanced by field assist. A need also exists for thermal transfer printing processes that enable the formation of high quality images on rough paper or other rough receiver sheets. An additional need exists for thermal transfer printing processes enhanced by field assist to enable formation of images wherein the solid areas are of uniform image density. Further, there is a need for thermal transfer printing processes enhanced by field assist to enable the formation of machine-readable magnetic characters.